FIG. 1 schematically shows a conventional example of a circuit I for controlling a load 2 (Q) supplied by a battery 3.
Control circuit 1 comprises a switch 11 (generally a MOS transistor) between a D.C. voltage (Vbat) terminal 12 and a terminal 13 of connection of load 2, the other terminal of the load being connected (generally directly) to ground and more generally to the other voltage (terminal 32) of the D.C. power supply than that to which terminal 12 is connected. Circuit 1 generally is an integrated circuit further comprising a terminal 14 of connection to ground and input-output terminals of data exchange (control signals, states, etc.) with a microcontroller 4 (μC). In the example shown, four terminals 15 to 18 of circuit 1, among which at least one input terminal of a (logic) turn-on control signal of switch 11, are provided.
A switch 5 symbolizes a general switch of the electric circuit, for example, actuated by the ignition key of a vehicle. Switch 5 connects positive terminal 31 of battery 3 to terminal 12.
Microcontroller 4 is generally supplied by a voltage regulator 6 (VR) receiving, via switch 5 and a diode 61 having its anode connected to the power switch, the battery voltage, and providing a regulated voltage to microcontroller 4.
Several electric protection elements further equip circuit 1 and its connections.
A first protection element is formed of a zener diode 19, internal to circuit 1, connecting terminals 12 and 14, the anode of diode 19 being connected to terminal 14. Diode 19 aims at protecting the portion of circuit I comprising the control elements of switch 11 by limiting the voltage of terminal 12.
A second element is formed of a diode 7 connecting terminal 14 to terminal 32, and thus to the ground or negative terminal of battery 3, the anode of diode 7 being connected to terminal 14. The function of diode 7 is to prevent a battery short-circuit in case of a polarity reversal. Diode 7 is on in normal operation and ensures the ground connection of circuit 1. It is blocked in case of a polarity reversal of the battery and then prevents diode 19 (in the on state) from connecting terminals 32 and 31, which would be the case if terminal 14 was directly connected to terminal 32.
Diode 7 may be replaced with a resistor that must then be of small value to avoid a significant voltage drop in normal operation. Such a small value however generates a significant dissipation in case of a polarity reversal of the battery.
Diode 7, which may be common to several circuits 1, independently from their consumption, and which generates an approximately constant voltage drop on the order of 0.6 volt, is thus conventionally preferred.
Microcontroller 4 is generally protected against a polarity reversal by diode 61.
Ground terminal 41 of the microcontroller must generally be directly connected to terminal 32. This, to enable it reading, without any voltage offset, data from sensors (not shown) directly connected to the battery “minus” 32.
A disadvantage of diode 7 is that it does not provide a protection of circuit 1 and of microcontroller 4 in case of transient negative pulses on supply terminal 12 (or 31). In the example of application to automobile, transient pulses correspond to normalized transient distrubances (ISO standard T/R 7637/1) of the battery voltage, of variable power. More generally, such transient pulses may be due, for example, to switchings of loads connected to the D.C. power supply.
Upon positive transient pulses on terminal 12, diode 7 remains on. Zener diode 19 then behaves as a voltage limiter and protects the control elements of switch 11.
However, upon negative transient pulses on terminal 12, diode 7 blocks. The internal ground (terminal 14) of circuit 1 then becomes strongly negative (with respect to the voltage of terminal 32). Now, the microcontroller has its ground referenced to terminal 32. As a result, inputs-outputs 15 to 18 follow the negative voltage of the pulse and draw current from the microcontroller. To overcome this phenomenon, each input-output 15 to 18 of circuit 1 is connected to the microcontroller by a protection resistor 45 to 48. The resistors must have a strong value (at least on the order of one KΩ) to ensure this protection function.
A disadvantage is linked to the presence of resistors 45 to 48 and to their bulk. In particular, the miniaturization objectives result in integrating series interfaces in the packages containing circuits 1 to limit connections to the microcontroller. The connections are then preferentially limited to a number of three (a dock connection, an output connection to the microcontroller, and an input connection from the microcontroller). It being series connections, the data interpretation is preferentially performed by digital words.
The presence of the protection resistors on the input-output lines poses another problem, particularly present for series communications, than that of the resistor size. Indeed, the resistors take part in RC cells (with the input or output capacitances, be they or not stray capacitances) that introduce a non-negligible delay in the data transmission between the microcontroller and the load control circuit(s).
In particular, in the application to the automobile field, the required response times are of at least one microsecond, which is incompatible with the use of series resistors on the order of one KΩ or more.